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Despite federal claims that most of the oil in the Gulf of Mexico is gone, some independent scientists said Wednesday that they fear the amount of crude left from the massive BP spill will wreak environmental destruction.

Among the unanswered questions is how much damage the remaining oil will have on marine life and coastlines. John Hocevar, a marine biologist with Greenpeace in Washington, said the answer may not be known for a year or longer.

"And it's more than losing a year of baby fish," Hocevar told msnbc.com. "We will eventually lose habitat. We will lose entire islands. ... It will have serious impact on our population of whales in the Gulf, the sea turtles, and it goes on. This is something we will be facing for 10 years to come."

Hocevar was reacting to a federal report released earlier in the day indicating that nearly three-quarters of the spilled oil — more than 152 million gallons — has been collected at the well by a temporary containment cap, been cleaned up or chemically dispersed, or naturally deteriorated, evaporated or dissolved.

That would leave 53 million gallons still in the Gulf, an amount nearly five times the size of the 11 million-gallon Exxon Valdez spill, which caused massive environmental damage in Alaska in 1989.

The government report was based on daily operational reports, estimates by scientists and analyses by federal experts. The government acknowledged it made certain assumptions about how oil dissolves in water naturally over time.

"This is a shaky report. The more I read it, the less satisfied I am with the thoroughness of the presentation," Florida State University oceanography professor Ian MacDonald told The Associated Press. "There are sweeping assumptions here."

NOAA chief Jane Lubchenco acknowledged the numbers could be off by as much as 10 percent. One of the scientists who peer-reviewed the work and is mentioned in the report, Ed Overton of Louisiana State University, said he wasn't comfortable with NOAA's putting precise percentages of how much oil is left in the Gulf. What would be more accurate would be a much broader range of, say, 40 million to 60 million gallons, he said.

Still, Overton thought the report was mostly good work. He said the Gulf itself deserves much of the credit, describing the body of water in two words: "incredibly resilient."

The White House claimed only 26 percent of the oil remained in the Gulf, but that was based on a 206-million-gallon figure for the spill that included oil that spewed from the pipe but was captured by BP and never got into the Gulf. Using the 172 million gallons that got into the Gulf, 31 percent of the oil remains.

So what happened to the oil?
Thank nature more than the federal government. Burning, skimming and chemically dispersing the spill got rid of 35 million gallons of oil, while natural processes of dispersion, evaporation and dissolving got rid of 84 million gallons, according to the report.

"Mother Nature is assisting here considerably," Lubchenco said. She cautioned that the oil that's left can harm wildlife for years or even decades to come, saying: "Diluted and out of sight doesn't necessarily mean benign."

Still, outside scientists said this was a just too-simple explanation for a complex oil that has confounded federal scientists at every turn.

"This is just way too neat," said Larry McKinney, director of the Texas A&M University research center on the Gulf of Mexico. "How can you even do this at this point? There's a lot of oil still floating out there."

McKinney said he most worried that this overly optimistic assessment would cost the government — and save BP — billions of dollars in the damage assessment process. McKinney, who has served as a state of Texas trustee in the process, said, "BP attorneys are placing this in plastic and putting this in frames."

White House energy adviser Carol Browner said, "We are going to continue to ensure BP is held accountable for damage they did."

MacDonald said the core of the idea here — that oil in water essentially has about a half-life of a week — makes sense, but what happened from there doesn't.

"There's some science here, but mostly, it's spin," he told The Associated Press. "And it breaks my heart to see them do it."

MacDonald pointed out that NOAA spent weeks sticking with its claim the BP well was spewing only 210,000 gallons a day. Now, after several revisions, the federal government said it really was 2.2 million gallons a day. So he has a hard time believing NOAA this time, he said.

'It's so bad'Thousands of birds and other animals are known to have been damaged or killed by the spill. Efforts are still under way in Louisiana, Mississippi, Alabama and Florida to clean up more than 600 miles of oiled shoreline. As of last weekend, the government and BP collected 35,818 tons of oily debris from shorelines but more work remains to be done.

"There is no reason to exaggerate the impact, because it's so bad that we don't have to exaggerate," Hocevar said. "I stepped into an area that was the size of a football field and it had 50,000 dead crabs. This is an area that can no longer sustain life. You don't want to see 50,000 of anything dead."

Adding to the controversy are questions over the environmental impact of the nearly 2 million gallons of chemical dispersants used by BP to help break the oil into into droplets so huge slicks wouldn't tarnish shorelines and coat marine animals, and to encourage the oil to degrade more quickly.

Lawmakers in Washington pressed scientists Wednesday to explain what effects those chemicals will have on the Gulf's ecosystem.

Paul Anastas of the EPA's office of research and development acknowledged there are "environmental tradeoffs" to consider when using dispersants but said earlier this week that "dispersants were less toxic than oil or oil-dispersants mixture."

Residents, however, remain skeptical.

"These dispersants are confounding our detection efforts," said Susan McMillan, a 30-year Florida resident and the founder of a nonprofit group called Protect Our Waters, which organized in response to the spill.

McMillan told The Associated Press she is curious about who is testing Florida's waters for dispersant levels, if anyone, and whether the chemicals are dangerous to people and marine life.

"We never want our Gulf to be used in a giant Frankenstein experiment again," she said.

Video: New Gulf controversy: Where’s the oil?

Closed captioning of: New Gulf controversy: Where’s the oil?

>>>to the oil disaster and the words we've all waited so long to hear. the
coast guard
said today that static kill operation to stop the oil flow is working and they do not expect any more oil to gush into the
gulf of mexico
. but there are plenty of questions about the government's claims about where all that oil went. nbc's chief environmental correspondent
anne thompson
joins us tonight from venice, louisiana. anne, good evening.

>> reporter: good evening,
lester
. bp says the static kill is working extremely well, but tonight there's a new controversy over just how much oil is left in the gulf. 107 days after the leak to poison the gulf began,
president obama
delivered some good news.

>>a report out today by our scientists show that the vast majority of the spilled oil has been dispersed or removed from the water.

>> reporter: but many of louisiana's fishermen who still can't fish because of the spill find the government's claims hard to believe.

>>i think they're jumping the gun way too quick.

>>they should have never put the dispersant on to begin with. they wanted the oil to sink because you can't see it.

>>they need to go to
orange beach
, alabama. it's pouring in.

>> reporter: the report says 33% of the oil was recovered, burned or chemically dispersed. 25% evaporated or dissolved. 16% broken down naturally into microscopic droplets and 26% is on or just below the surface, washed ashore or collected from shore. that's more than a million barrels, four times the size of the
exxon valdez spill
. it is far less than smt doom's day predictions some say have happened but no reason to stand down.

>>we want to be very, very clear that this does not mean there is more to be done. there remains a lot to be done.

>> reporter: oil spills
can take years to reveal their impact.

>>we can't stop pursuing the answers.

>> reporter: as alaska's chief environmentalist during the exxon spill, he saw the collapse of the herring in
prince william sound
.

>>the documentation of the crash in the fishery was two years after the spill, the
exxon valdez spill
.

>> reporter: so just because things are good today doesn't mean they'll be good a year from now?

>>that's right.

>> reporter: what is good today is the static kill. so far the heavy mud is holding back the oil giving the energy secretary confidence about the well.

>>what we've seen is consistent with an undamaged well, that's good news.

>> reporter: it certainly is. now, the next decision secretary chu, his team of scientists and bp has to make is whether to cement the well from the top or just do it from the bottom with the relief well.
lester
.

>>anne, if so much of that oil has in fact disappeared, that's good news. at the same time, it raises more questions about these dispersants that were used. could that prevent some of these areas from being fished into the near future?

>> reporter: well, they don't know that yet,
lester
. they say only 8% of all the oil that spilled was actually dispersed chemically by that, you know, though things that came down. but that is the big question. there's so much we don't know. and what are the long-term impacts of using that dispersant. how does it break down, what does it do to the
food chain
. those are all questions we're
still waiting
answers on.

The Blue Dolphin, left, and the HOS Centerline, the ships supplying the mud for the static kill operation on the Helix Q4000, are seen delivering mud through hoses at the site of the Deepwater Horizon Oil Spill in the Gulf of Mexico, off the coast of Louisiana, on Aug. 3, 2010. In the background is the Development Driller III, which is drilling the primary relief well.
(Gerald Herbert / AP)
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Eddie Forsythe and Don Rorabough dump a box of blue crabs onto a sorting table at B.K. Seafood in Yscloskey, La., on Aug. 3, 2010. The crabs were caught by fisherman Garet Mones. Commercial and recreational fishing has resumed, with some restrictions in areas that were closed by the Deepwater Horizon oil spill.
(Chuck Cook / AP)
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Sea turtle hatchlings that emerged from eggs gathered on the northern Gulf Coast of Florida are released at Playalinda Beach on the Canaveral National Seashore near Titusville, Fla., on Aug. 2, 2010. The sea turtles were born at a Kennedy Space Center incubation site, where thousands of eggs collected from Florida and Alabama beaches along the Gulf of Mexico have been sent.
(Craig Rubadoux / Florida Today via AP)
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A crab, covered with oil, walks along an oil absorbent boom near roso-cane reeds at the South Pass of the Mississippi River in Plaquemines Parish, Louisiana on Aug. 1, 2010. BP is testing the well to see if it can withstand a "static kill" which would close the well permanently.
(Pool / Reuters)
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A boat motors through a sunset oil sheen off East Grand Terre Island, where the Gulf of Mexico meets Barataria Bay on the La. coast, on the evening of July 31.
(Gerald Herbert / AP)
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Oil approaches a line of barges and boom positioned to protect East Grand Terre Island, partially seen at top right, on July 31.
(Gerald Herbert / AP)
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Oil from the Deepwater Horizon oil spill is seen near an unprotected island in the Gulf of Mexico near Timbalier Bay, off the coast of Louisiana on Wednesday, July 28.
(Gerald Herbert / AP)
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Greenpeace activists stand outside a BP gas station in London, England, on July 27 after they put up a fence to cut off access. Several dozen BP stations in London were temporarily shut down to protest the Gulf spill.
(Leon Neal / AFP - Getty Images)
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James Wilson sells T-shirts to those arriving in Grand Isle, La., for the music festival Island Aid 2010 on July 24.
(Dave Martin / AP)
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Activists covered in food coloring made to look like oil protest BP's Gulf oil spill in Mexico City on July 22. The sign at far left reads in Spanish "Petroleum kills animals."
(Alexandre Meneghini / AP)
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People in Lafayette, La., wear "Keep Drilling" tee shirts at the "Rally for Economic Survival" opposing the federal ban on deepwater drilling in the Gulf of Mexico on Wednesday, July 21. Supporters at the rally want President Obama to lift the moratorium immediately to protect Louisiana's jobs and economy.
(Ann Heisenfelt / EPA)
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A flock of white ibis lift off from marsh grass on Dry Bread Island in St. Bernard Parish, La., July 21. Crews found about 130 dead birds and 15 live birds affected by oil from the Deepwater Horizon oil spill on July 19 in the eastern part of the parish behind the Chandeleur Islands.
(Patrick Semansky / AP)
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Kenneth Feinberg, administrator of the BP Oil Spill Victim Compensation Fund testifies during a hearing before the House Judiciary Committee on July 21 in Washington, D.C. The hearing was to examine the claim process for victims of the Gulf Coast oil spill.
(Alex Wong / Getty Images)
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An American white pelican has its wings checked during a physical examination at Brookfield Zoo’s Animal Hospital by Michael Adkesson and Michael O’Neill on July 21. The bird, along with four other pelicans, was rescued from the Gulf Coast oil spill and will be placed on permanent exhibit at the zoo.
(Jim Schulz / Chicago Zoological Society via AP)
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Native people of the Gwich'in Nation form a human banner on the banks of the Porcupine River near Ft. Yukon, Alaska July 21, in regard to the BP oil spill with a message to protect the Arctic National Wildlife Refuge from oil development. The images include a Porcupine caribou antler and a threatened Yukon River Salmon.
(Camila Roy / Spectral Q via Reuters)
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Editor's note:
This image contains graphic content that some viewers may find disturbing.